There is much speculation as to how Vikings found their way from Scandinavia to America. Scientists are just as unsure as to how the garden warbler migrates from Sub-Saharan Africa to England without getting lost or how exploring bees return to the hive. Polarized light may be the answer to both questions. The Viking connection was first suggested by Thorkild Ramskou, a Danish archaeologist.

Scientists have long tried to equate the Viking sólarsteinn, or sunstone of legend, with something we could identify. Ramskou thought that Vikings might have used the polarization of the skylight to help them navigate when the sun was hidden behind clouds. In the case of the Vikings, they possibly used one of two crystals, cordierite, also called lolite, of Norway, or calcite of Iceland.

By holding a polarizing crystal up to the sky and rotating it, you can determine the direction of the Sun. Summer, when sailors far north ventured upon the seas, they were confronted with nearly perpetual daylight, preventing them from using the stars to navigate. Light consists of electromagnetic waves that oscillate perpendicular to the direction of the light’s travel. When the oscillations all point in the same direction, the light is polarized.

A polarizing crystal allows only light polarized in certain directions to pass through it. With an appropriately shaped crystal, you can tell the direction of skylight polarization because its color will change (known as pleochroism), from blue to light yellow for example when it’s pointing towards the sun.

Experiments with birds indicate that the skylight polarization pattern near the horizon at sunrise and sunset provides them with a season-and-latitude-independent compass calibration reference. Migratory birds are also born with an innate magnetic compass preference that coincides with their species’ migratory direction.

Iron-containing sensory dendrites in the inner dermal lining of the upper beak may serve as an avian magnetometer system. The migratory garden warbler Sylvia borin, as well as the non-migratory farm bird that provides eggs for your morning breakfast, sport such a mechanism.

The Vikings, however, had no access to a magnetic compass in 1000AD, and even if they had it would have been of little use so near to the North Pole. Two scientists have been testing the assumptions related to polarized light for half a decade. Gábor Horváth, an optics researcher at Eötvös University in Budapest, and Susanne Åkesson, a migration ecologist from Lund University, Sweden, reviewed their findings in the Philosophical Transactions of the Royal Society B.


Garden Warbler. Credit: Aron Tanti

Unfortunately, until solid archaeological evidence surfaces, such as a polarizing crystal lying on the deck of an ancient, deteriorating Viking ship, this all remains academic speculation. The persisting legend and gems made from the so-called Viking Compass are illustrated in the video below.